Breaking the Rules: Exploring Violation of the Kirchhoff Radiation Law in Experiments

The understanding of emission of thermal radiation has been shaped by the Kirchhoff thermal radiation law, which asserts equality of the absorptivity and emissivity of a given object for each wavelength, direction, and polarization. This equality is built on reciprocity; and the use of reciprocal materials to demonstrate spatial and temporal coherence of thermal emission has proven a landmark in the nanophotonics community. We have experimentally measured inequality of the spectral emissivity and spectral absorptivity of a thermal emitter, and thus directly observe a violation of Kirchhoff’s law. We achieve this by measuring the directional thermal emissivity and absorptivity from a nanophotonic structure in a magnetic field, which breaks the reciprocity conditions. We not only observe inequality in emissivity and absorptivity, but also demonstrate a new relationship between the two for the non-reciprocal photonic structure. Thermal emission phenomenon that can break free of Kirchhoff’s law are of fundamental interest and may open a multitude of new applications; we will discuss further directions for experimental exploration of related phenomena.